Fabrication method of in-plane switching mode liquid crystal display device including twisted nematic liquid crystal layer between first and second ferroelectric liquid crystal layers

ABSTRACT

An in-plane switching mode liquid crystal display device includes a first substrate including a pixel electrode in a pixel region, a second substrate facing the first substrate and including a common electrode, a first alignment layer on the pixel electrode, a second alignment layer on the common electrode, a first ferroelectric liquid crystal layer on the first alignment layer and including a first spontaneous polarization, a second ferroelectric liquid crystal layer on the second alignment layer and including a second spontaneous polarization, a rotational direction of the first ferroelectric liquid crystal layer with respect to the first alignment layer being different from a rotational direction of the second ferroelectric liquid crystal layer with respect to the second alignment layer, and a twisted nematic liquid crystal layer between the first and second ferroelectric liquid crystal layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of application Ser. No. 11,159,300,filed Jun. 23, 2005 now U.S. Pat. No. 7,787,088, now allowed whichclaims priority to Korean Patent Application No. 10-2004-0116898, filedDec. 30, 2004, all of which are hereby incorporated by reference for allpurposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device and,more particularly, an in-plane switching mode liquid crystal displaydevice and a fabrication method thereof that provide a fast responsespeed and an improved transmittance.

2. Discussion of the Related Art

As the information-oriented age is advancing, display devices fordisposing and displaying information are actively being developed. Moreparticularly, a flat panel display device, e.g., a liquid crystaldisplay (LCD) device, having a small thickness, lightness weight and alow power consumption, has been actively studied. An LCD device usesoptical anisotropy and birefringence characteristics of liquid crystalmolecules in a liquid crystal layer to display images.

An LCD device may be an active matrix type liquid crystal display devicehaving a thin film transistor and a pixel electrode connected to thethin film transistor arranged in a matrix manner. For example, an LCDdevice includes an upper substrate, a lower substrate, and a liquidcrystal layer disposed between the upper and lower substrates. The uppersubstrate is referred to as a color filer substrate, and a lowersubstrate is referred to as an array substrate. When a driving voltageis supplied to a common electrode on the upper substrate and a pixelelectrode on the lower substrate, a perpendicular electric field isformed between the common electrode and the pixel electrode. Because theliquid crystal molecules in the liquid crystal layer are thin and long,and have a pretilt angle, the pretilt angle is changed by the electricfield. Thus, an arranging direction of the liquid crystal molecules ischanged, thereby altering the optical anisotropic of the liquid crystalmolecules and displaying images.

However, when the liquid crystal layer is driven by the perpendicularelectric field, a transmittance and an aperture ratio increase but aviewing angle decreases. Accordingly, to solve this disadvantage, adriving method of liquid crystal by in-plane switching (IPS) using ahorizontal electric field has been suggested.

FIG. 1 is a cross-sectional view illustrating an in-plane switching modeliquid crystal display device according to the related art. In FIG. 1, aliquid crystal panel of an in-plane switching mode liquid crystaldisplay device includes a color filter substrate 9 having a colorfilter, an array substrate 10 having a thin film transistor facing thecolor filter substrate 9, and a liquid crystal layer 11 disposed betweenthe color filter substrate 9 and the array substrate 10. A commonelectrode 17 and a pixel electrode 30 are disposed in parallel to eachother on the array substrate 10, and a horizontal electric field L isformed by a difference in voltages supplied to the common electrode 17and the pixel electrode 30. Thus, the in-plane switching mode liquidcrystal display device is driven by using the horizontal electric fieldL to control liquid crystal molecules.

FIGS. 2A and 2B are cross-sectional views illustrating ‘off’ and ‘on’states of the in-plane switching mode liquid crystal display deviceshown in FIG. 1. As shown in FIG. 2A, in an ‘off’ state, when novoltages are supplied to the common electrode 17 and the pixel electrode30, no horizontal electric field is formed. Thus, all liquid crystalmolecules 11 are aligned in the same direction.

As shown in FIG. 2B, in an ‘on’ state, when voltages are supplied to thecommon electrode 17 and the pixel electrode 30, the horizontal electricfield L is formed. Locations of liquid crystal molecules 11 a that arelocated corresponding to the common electrode 17 and the pixel electrode30 remain unchanged by the horizontal electric field L. However, liquidcrystal molecules 11 b that are located between the common electrode 17and the pixel electrode 30 become aligned in the same direction as thehorizontal electric field L. Accordingly, the in-plane switching modeliquid crystal display device has a broad viewing angle, because theliquid crystal moves by the horizontal electric field. As a result, thein-plane switching mode liquid crystal display device may be viewed indirection of above/below/left/right of about 80° to 85° without areversal process.

However, since the common electrode and the pixel electrode are formedon the same substrate in a pixel region, they shield the pixel region,thereby decreasing an aperture ratio. In addition, since a lightquantity passing through the liquid crystal display device is limited, abrightness decreases.

In addition, the general liquid crystal display device (TN mode liquidcrystal display device) and the in-plane switching mode liquid crystaldisplay device use a twisted nematic liquid crystal. Since the twistednematic liquid crystal has a response time over 30 ms (i.e., a slowresponse speed), the liquid crystal display device using a twistednematic liquid crystal has a problem of low display quality in that anafterimage occurs when implementing an animation or fast movements. Toimprove these problem of the response speed, an FLC mode liquid crystaldisplay device using a ferroelectric liquid crystal having a superiorresponse property has been developed.

The ferroelectric liquid crystal is often referred to as a chiralsmectic C liquid crystal having a response time below many ms. In otherwords, a response speed of the ferroelectric liquid crystal molecules isfast. In particular, each layer of the chiral smectic C liquid crystalis arranged with an angle. When an electric field is supplied to thechiral smectic C liquid crystal, a dipole moment is arranged in onedirection, and a molecular alignment is uniform and maintained after theelectric field is eliminated. Further, when an electric field issupplied in an opposite direction of the chiral smectic C liquidcrystal, the molecular alignment may be reversed in an oppositedirection at high speed. Thus, the molecular alignment of theferroelectric liquid crystal differs according to a polarization of anelectric field, and the FLC mode liquid crystal display device has afast response property.

FIG. 3 is a cross-sectional view illustrating an FLC mode liquid crystaldisplay device using a ferroelectric liquid crystal according to therelated art. In FIG. 3, a liquid crystal panel 40 of an FLC mode liquidcrystal display device includes a ferroelectric liquid crystal layer 80in a gap d1 between a first alignment layer 55 on an array substrate 50and a second alignment layer 75 on a color filter substrate 70. Theferroelectric liquid crystal layer 80 includes a plurality offerroelectric liquid crystal molecules 82 arranged with an angle in thegap d1. The gap d1 is generally smaller than 2 μm.

Thus, the FLC mode liquid crystal display device has problems in thatsince the gap between the substrates is below 2 μm and since theferroelectric liquid crystal molecules are in almost a gel state at anormal temperature, it is difficult to inject the ferroelectric liquidcrystal molecules in the gap. In addition, because the gap is small, itis also difficult to provide a wider degree of control of theferroelectric liquid crystal molecules using a supplied electric field.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an in-plane switchingmode liquid crystal display device and a fabrication method thereof thatsubstantially obviate one or more of the problems due to limitations anddisadvantages of the related art.

An object of the present invention is to provide an in-plane switchingmode liquid crystal display device and a fabrication method thereof thatimprove an aperture ratio and a transmittance.

Another object of the present invention is to provide an in-planeswitching mode liquid crystal display device and a fabrication methodthereof that improve a response speed, widen a viewing angle andincrease brightness.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, an in-planeswitching mode liquid crystal display device includes a first substrateincluding a pixel electrode in a pixel region, a second substrate facingthe first substrate and including a common electrode, a first alignmentlayer on the pixel electrode, a second alignment layer on the commonelectrode, a first ferroelectric liquid crystal layer on the firstalignment layer and including a first spontaneous polarization, a secondferroelectric liquid crystal layer on the second alignment layer andincluding a second spontaneous polarization, a rotational direction ofthe first ferroelectric liquid crystal layer with respect to the firstalignment layer being different from a rotational direction of thesecond ferroelectric liquid crystal layer with respect to the secondalignment layer, and a twisted nematic liquid crystal layer between thefirst and second ferroelectric liquid crystal layers.

In another aspect, a fabrication method of an in-plane switching modeliquid crystal display device includes forming a pixel electrode on afirst substrate, forming a common electrode on a second substrate,forming a first alignment layer on the pixel electrode, forming a secondalignment layer on the common electrode, forming a first ferroelectricliquid crystal layer on the first alignment layer, forming a secondferroelectric liquid crystal layer on the second alignment layer,exposing the first ferroelectric liquid crystal layer to a firstatmosphere to generate a first spontaneous polarization, exposing thesecond ferroelectric liquid crystal layer to a second atmospheredifferent from the first atmosphere to generate a second spontaneouspolarization, a rotational direction of the first ferroelectric liquidcrystal layer with respect to the first alignment layer being differentfrom a rotational direction of the second ferroelectric liquid crystallayer with respect to the second alignment layer, attaching the firstand second substrates with the first and second ferroelectric liquidcrystal layers facing each other, and forming a twisted nematic liquidcrystal layer between the first and second ferroelectric liquid crystallayers.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a cross-sectional view illustrating an in-plane switching modeliquid crystal display device according to the related art;

FIGS. 2A and 2B are cross-sectional views illustrating ‘off’ and ‘on’states of the in-plane switching mode liquid crystal display deviceshown in FIG. 1;

FIG. 3 is a cross-sectional view illustrating an FLC mode liquid crystaldisplay device using a ferroelectric liquid crystal according to therelated art;

FIG. 4 is a view illustrating movements of ferroelectric liquid crystaldirectors by a supplied electric field;

FIGS. 5A to 5C are cross-sectional views illustrating a process offorming a ferroelectric liquid crystal layer of an in-plane switchingmode liquid crystal display device according to a first exemplaryembodiment of the present invention;

FIG. 6 is a cross-sectional view illustrating one pixel region of anin-plane switching mode liquid crystal display device according to thefirst exemplary embodiment of the present invention;

FIGS. 7A to 7C are cross-sectional views illustrating a process offorming a ferroelectric liquid crystal layer on an array substrate of anin-plane switching mode liquid crystal display device according to asecond exemplary embodiment of the present invention;

FIGS. 8A to 8C are cross-sectional views illustrating a process offorming a ferroelectric liquid crystal layer on a color filter substrateof an in-plane switching mode liquid crystal display device according tothe second exemplary embodiment of the present invention;

FIG. 9 is a cross-sectional view illustrating one pixel region of anin-plane switching mode liquid crystal display device according to thesecond exemplary embodiment of the present invention; and

FIG. 10 is a graph illustrating a relation of a transmittance and asupplied voltage to an in-plane switching mode liquid crystal displaydevice according to the related art and the first and second exemplaryembodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

In general, a ferroelectric liquid crystal has a permanent dipole momentin a particular direction even without an exterior electric field. Oneend of a ferroelectric liquid crystal director has a circle movement inone direction with another end of the ferroelectric liquid crystaldirector being fixed.

FIG. 4 is a view illustrating movements of ferroelectric liquid crystaldirectors by a supplied electric field. As shown in FIG. 4, one end of aferroelectric liquid crystal director 106 is fixed at an apex of a cone100, which illustrates a movement locus of the ferroelectric liquidcrystal director 106. Another end of the ferroelectric liquid crystaldirector 106 rotates in the one direction in a circumference phase of aspiral. For example, when an electric field is supplied, theferroelectric liquid crystal director 106 rotates in the direction of aspontaneous polarization 103.

In general, a liquid crystal has a phase transition in accordance with atemperature. In other words, a ferroelectric liquid crystal has thefollowing phase transitions: an isotropic phase transiting to a nematicphase (N*); the nematic phase (N*) transiting to a smectic phase (SmC*);and the smectic phase (SmC*) transiting to a crystal phase as thetemperature changes from a high temperature to a low temperature.Further, a viscosity of the ferroelectric liquid crystal also changesaccording to the temperature. In other words, the viscosity is thelowest in the isotropic phase and the highest in the crystal phase.Accordingly, in order to form the ferroelectric liquid crystal on asubstrate, the ferroelectric liquid crystal should be the isotropicphase state having the low viscosity and then be heated.

After a liquid crystal panel is formed, a phase used mainly in a normaltemperature is smectic phase (SmC*), and a spontaneous polarization isrevealed when the nematic phase (N*) transits to the smectic phase(SmC*). The spontaneous polarization controls a rotation of the liquidcrystal director when a voltage is supplied thereto. When the liquidcrystal director moves in the one direction by the spontaneouspolarization, the rotation direction is continuously maintained.

In addition, when the ferroelectric liquid crystal made of the highmolecular substance is used as a dynamic alignment layer made of a highmolecular substance of a nematic liquid crystal, there is problems suchthat an adhesive strength from an interface and a rotatory power of theliquid crystal director decrease by an interaction of the high molecularsubstances. Accordingly, the ferroelectric liquid crystal according toan embodiment of the present invention has a stable alignment in theinterface from the alignment layer since the ferroelectric liquidcrystal is made of the monomolecular substance.

FIGS. 5A to 5C are cross-sectional views illustrating a process offorming a ferroelectric liquid crystal layer of an in-plane switchingmode liquid crystal display device according to a first exemplaryembodiment of the present invention. As shown in FIG. 5A, a polyimide(PI) of a high molecular substance is spread on an array substrate 110.The array substrate 110 includes a thin film transistor Tr and a pixelelectrode 120, and may be subsequently attached to a color filtersubstrate (not shown) including red, green and blue color filters and acommon electrode. The array substrate 110 further includes an alignmentlayer 122 on the thin film transistor Tr and the pixel electrode 120 foraligning a ferroelectric liquid crystal layer formed thereon.

A ferroelectric liquid crystal layer of an isotropic phase 130, which ismade of a monomolecular substance and has a low viscosity by heatingwith a high temperature, is coated on the alignment layer 122 with athickness of below 1 μm. For example, a thickness of about 1000 Å to3000 Å may be coated on the alignment layer 122. The coating may becarried out by using a spin coating device (not shown) and a slitcoating device (not shown) and may include adding a solvent for loweringthe viscosity.

As shown in FIG. 5B, the ferroelectric liquid crystal layer of theisotropic phase 130 (shown in FIG. 5A) transits to a ferroelectricliquid crystal layer of a nematic phase (N*) 131 as the temperaturedecreases slowly. Further, as shown in FIG. 5C, the ferroelectric liquidcrystal layer of the nematic phase (N*) 131 (shown in FIG. 5B) transitsto a ferroelectric liquid crystal layer of a smectic phase (SmC*) 133 asthe temperature further decreases.

When the ferroelectric liquid crystal layer of the nematic phase (N*)131 transits to the ferroelectric liquid crystal layer of the smecticphases (SmC*) 133, by supplying + or − electric field or exposing toair, a spontaneous polarization PS having a rotational direction to thealignment layer is formed. In particular, the ferroelectric liquidcrystal of the nematic phase (N*) 131 is contacted to the alignmentlayer 122, which is a polarity medium, instead of air, which is anon-polarity medium.

Since air is a non-polarity medium and the alignment layer i.e.,polyimide (PI) is the polarity medium, the spontaneous polarization PSis revealed with direction of the polarity medium when the ferroelectricliquid crystal of the nematic phase (N*) transits to the ferroelectricliquid crystal of the smectic phase (SmC*). Further, since thespontaneous polarization PS has a high polarity, the liquid crystalmolecules revealed the spontaneous polarization PS is arranged in theone direction. Also, when the + or − electric field is supplied, theferroelectric liquid crystal director 135 rotates or is fixed whilemaintaining the direction by the spontaneous polarization PS revealedwith direction to the alignment layer. When the polarization of theelectric field accords with property of the revealed spontaneouspolarization, the ferroelectric liquid crystal director rotates in thedirection of the spontaneous polarization. When the polarization of theelectric field differs with property of the revealed spontaneouspolarization, the ferroelectric liquid crystal director is fixed.

A fabricating method of the array substrate and the color filtersubstrate of the liquid crystal display device having the ferroelectricliquid crystal of the smectic phase (SmC*) is the same as a fabricatingmethod of general liquid crystal display device. For example, a sealantof adhesion may be formed on a rim of one of the array substrate and thecolor filter substrate, and then the two substrates having ferroelectricliquid crystal layers of the smectic phase (SmC*) are attached facingeach other. Then, the twisted nematic liquid crystal is injected betweenthe two substrates, and thus the in-plane switching mode liquid crystaldisplay device is formed.

As the temperature changes from a high temperature to a low temperature,the twisted nematic liquid crystal has the following phase transitions:an isotropic phase transiting to a nematic phase (N*); the nematic phase(N*) transiting to a smectic phase (SmC*); and the smectic phase (SmC*)further transiting to a crystal phase. Since the twisted nematic liquidcrystal has the nematic phase (N*) in the normal temperature and theferroelectric liquid crystal has the smectic phase (SmC*) in the normaltemperature, the twisted nematic liquid crystal and the ferroelectricliquid crystal are not mixed or fused.

FIG. 6 is a cross-sectional view illustrating one pixel region of anin-plane switching mode liquid crystal display device according to thefirst exemplary embodiment of the present invention. Although not shown,a plurality of pixel regions are defined by crossing of data and gatelines on an array substrate 110, and a switching element and a pixelelectrode connected the switching element are disposed in each pixelregion. The array substrate 110 also includes a first alignment layer122 disposed on the entire surface of the array substrate 110, and afirst ferroelectric liquid crystal layer of smectic phase (SmC*) 133disposed on the first alignment layer 122. The first ferroelectricliquid crystal layer of smectic phase (SmC*) 133 may have a thickness ofabout 1000 Å to 3000 Å. A first spontaneous polarization Ps1 of a liquidcrystal director 135 in the first ferroelectric liquid crystal layer 133rotates in the direction of the first alignment layer 122.

Although not shown, a plurality of color filter patterns, e.g., red,green and blue color filter patterns, are disposed on a color filtersubstrate 170 and corresponding to each pixel region. A black matrix(not shown) is disposed in a boundary of each color filter pattern, anda common electrode (not shown) is disposed on the color filter patternsand the black matrix. The color filter substrate 170 also includes asecond alignment layer 175 on the common electrode (not shown), and asecond ferroelectric liquid crystal layer of a smectic phase (SmC*) 180disposed on the second alignment layer 175. The second ferroelectricliquid crystal layer of smectic phase (SmC*) may have a thickness of1000 Å to 3000 Å and a thickness the same as the first ferroelectricliquid crystal layer of smectic phase (SmC*) 133. A spontaneouspolarization Ps2 of a liquid crystal director 182 in the secondferroelectric liquid crystal layer 180 rotates in the direction of thesecond alignment layer 175.

In addition, a twisted nematic liquid crystal layer 190 is disposedbetween the first and second ferroelectric liquid crystal layers 133 and180. The twisted nematic liquid crystal layer 190 moves according to asupplied electric field, with the first and second ferroelectric liquidcrystal layers 133 and 180 acting as a dynamic alignment layer.

For example, as a perpendicular electric field is generated between thepixel electrode (not shown) of the array substrate 110 and the commonelectrode (not shown) of the color filter substrate 170, the first andsecond ferroelectric liquid crystal directors 135 and 182 in the firstand second ferroelectric liquid crystal layers 133 and 180 are rotatedby the perpendicular electric field. Simultaneously the first and secondferroelectric liquid crystal layers 133 and 180 rotate to left and rightdirections according to the rotation of the liquid crystal directors 135and 182. As a result, the first and second ferroelectric liquid crystallayers 133 and 180 act as the dynamic alignment layer for the twistednematic liquid crystal layer 190. As a result, the twisted nematicliquid crystal layer 190 moves as if the horizontal electric field issupplied. Accordingly, the in-plane switching mode liquid crystaldisplay device according to an embodiment of the present invention has abroad viewing angle, and an increased brightness, without decreasing anaperture ratio.

The movement of the nematic liquid crystal according to suppliedelectric field will be explained in more detail. When a DC voltage issupplied to a liquid crystal for many times, a degradation occurs.Accordingly, in order to prevent this degradation, the liquid crystaldisplay device makes a driving by changing a polarization of a suppliedvoltage periodically. In other words, an inverted voltage of + and −polarity periodically is supplied to the pixel electrode in accordancewith the common electrode standardized. When the + and − polarity isreversely supplied, the director of the ferroelectric liquid crystal ofsmectic phase (SmC*) having a spontaneous polarization of a specificrotational direction rotates in the rotational direction of thespontaneous polarization when the direction of the spontaneouspolarization accords with the polarity of the electric field. Inaddition, the director of the ferroelectric liquid crystal is fixed whenthe direction of the spontaneous polarization do not accord with thepolarity of the electric field.

Accordingly, since the spontaneous polarization Ps2 of the secondferroelectric liquid crystal layer 180 and the spontaneous polarizationPs1 of the first ferroelectric liquid crystal layer 133 are facing thesecond alignment layer 175 and the first alignment layer 122,respectively, when + electric field (for one example, when a voltage ofpixel electrode is high, a voltage of common electrode is low) issupplied, the liquid crystal director 182 in the second ferroelectricliquid crystal layer 175 of the color filter substrate 170 is fixed andthe liquid crystal director 135 in the first ferroelectric liquidcrystal layer 133 of the array substrate 110 rotates. As the firstferroelectric liquid crystal layer 133 rotates fast dynamically, thetwisted nematic liquid crystal molecules located on near the firstferroelectric liquid crystal layer 133 rotate more fast than generalrotational velocity of the twisted nematic liquid crystal, thus aresponse speed increases.

However, since the direction of the spontaneous polarization of theferroelectric liquid crystal layer is opposite to each other, thein-plane switching mode liquid crystal display device according to thefirst exemplary of the present invention does not control fully a wholeliquid crystal of the twisted nematic liquid crystal layer. Thus, theliquid crystal directors of the one ferroelectric liquid crystal layerare fixed without a rotation as when the voltage is not supplied, andthe liquid crystal directors of another ferroelectric liquid crystallayer rotate when + or − electric field is supplied. Accordingly, anin-plane switching mode liquid crystal display device according to asecond exemplary embodiment of the present invention offers such thatthe twisted nematic liquid crystal is more effectively controlled.

The second embodiment of the present invention has structure that bothferroelectric liquid crystal layers on an array substrate and a colorfilter substrate rotate when + or − electric field is supplied, in otherwords, the rotational direction of the two spontaneous polarizationsaccords. As explained in the first exemplary embodiment, theferroelectric liquid crystal has a phase transition, in particular, thespontaneous polarization is revealed when the nematic phase (N*) istransit to the smectic phase (SmC*). When the spontaneous polarizationis reveled, a direction of the spontaneous polarization is determined inthe direction of a supplied electric field when + or − electric field issupplied, and in the direction of a polarity substance when a polaritysubstance and a non-polarity substance exist. Accordingly, the secondexemplary embodiment has a more superior property of a response speedthan the first exemplary embodiment, which has a structure that oneliquid crystal director is fixed and another liquid crystal directorrotates, thereby the ferroelectric liquid crystal layer having astructure that the both liquid crystal directors in the ferroelectricliquid crystal layers rotate in the one direction is formed.

FIGS. 7A to 7C are cross-sectional views illustrating a process offorming a ferroelectric liquid crystal layer on an array substrate of anin-plane switching mode liquid crystal display device according to asecond exemplary embodiment of the present invention. As shown in FIG.7A, an array substrate 210 includes a thin film transistor Tr, a pixelelectrode 220, and a first alignment layer 222 on the thin filmtransistor Tr and the pixel electrode 220. A first ferroelectric liquidcrystal layer of an isotropic phase 230, which is made of amonomolecular substance and has a low viscosity by heating with a hightemperature, is coated on the first alignment layer 222 with a thicknessof below 1 μm. For example, a thickness of about 1000 Å to 3000 Å may becoated on the first alignment layer 222.

As shown in FIG. 7B, the first ferroelectric liquid crystal layer of theisotropic phase 230 (shown in FIG. 7A) transits to a ferroelectricliquid crystal layer of a nematic phase (N*) 231 as the temperaturedecreases slowly. Further, as shown in FIG. 7C, the ferroelectric liquidcrystal layer of the nematic phase (N*) 231 (shown in FIG. 7B) transitsto a ferroelectric liquid crystal layer of a smectic phase (SmC*) 233 asthe temperature further decreases. A liquid crystal director 235 rotatesin the direction of the first alignment layer 222 by revealing of afirst spontaneous polarization Ps1.

FIGS. 8A to 8C are cross-sectional views illustrating a process offorming a ferroelectric liquid crystal layer on a color filter substrateof an in-plane switching mode liquid crystal display device according tothe second exemplary embodiment of the present invention. As shown inFIG. 8A, a color filter substrate 270 includes color filter layer 271, acommon electrode 272, and a second alignment layer 275. The color filterlayer 271 includes a plurality of red sub-color filter patterns 271 a,green sub-color filter pattern 271 b and blue sub-color filter patterns271 c. A second ferroelectric liquid crystal layer of an isotropic phase277, which is made of a monomolecular substance and has a low viscosityby heating with a high temperature, is coated on the second alignmentlayer 275 with a thickness of below 1 μm. For example, a thickness ofabout 1000 Å to 3000 Å may be coated on the second alignment layer 275,and the thickness of the second ferroelectric liquid crystal layer ofthe isotropic phase 277 may be the same as the first ferroelectricliquid crystal layer of the isotropic phase 230 (shown in FIG. 7A).

As shown in FIG. 8B, the ferroelectric liquid crystal layer of theisotropic phase 277 (shown in FIG. 8A) transits to a ferroelectricliquid crystal layer of a nematic phase (N*) 278 as the temperaturedecreases slowly. Further, as shown in FIG. 8C, the ferroelectric liquidcrystal layer of the nematic phase (N*) 278 (shown in FIG. 8B) transitsto a ferroelectric liquid crystal layer of a smectic phase (SmC*) 280 asthe temperature further decreases. A liquid crystal director 282 rotatesin the direction of an O₂ atmosphere by revealing of a secondspontaneous polarization Ps2.

When the ferroelectric liquid crystal layer of the nematic phase (N*)278 transits to a ferroelectric liquid crystal layer of a smectic phase(SmC*) 280, an electric field is supplied on the ferroelectric liquidcrystal layer. Alternatively, the ferroelectric liquid crystal layer isexposed in an atmosphere having higher polarity than the secondalignment layer 275 made of polyimide (PI). For example, theferroelectric liquid crystal of nematic phase (N*) 278 transits to theferroelectric liquid crystal of the smectic phase (SmC*) 280 accordingto temperature decrease in an O₂ atmosphere, which has higher polaritythan air. Accordingly, when the ferroelectric liquid crystal thereof isexposed in the O₂ atmosphere, the revealed spontaneous polarization Ps2rotates in the direction of substance with the high polarity. As aresult, contrary to the array substrate 210 (shown in FIG. 7C), whichthe direction of the first spontaneous polarization Ps1 faces to thefirst alignment layer 222, the direction of the second spontaneouspolarization Ps2 faces to the O₂ atmosphere not to the second alignmentlayer 275.

Further, although not shown, a twisted nematic liquid crystal isdisposed between the first and second ferroelectric liquid crystallayers of the array substrate (210 of FIG. 7C) and the color filtersubstrate (270 of FIG. 8C). Then, a patterned sealant is formed on a rimof one of the two substrates, and then the two substrates are attachedeach other. Accordingly, the in-plane switching mode liquid crystaldisplay device according to the second exemplary embodiment of thepresent invention is formed.

FIG. 9 is a cross-sectional view illustrating one pixel region of anin-plane switching mode liquid crystal display device according to thesecond exemplary embodiment of the present invention. As shown in FIG.9, the rotational directions of the spontaneous polarizations Ps1, Ps2with respect to the array substrate 210 and the color filter substrate270, respectively, are opposite to each other. Accordingly, when thearray substrate 210 and the color filter substrate 270 are attachedfacing each other, both the directions of the spontaneous polarizationsPs1, Ps2 face to the first alignment layer 222 of the array substrate210. For example, the rotational directions of the rotational directionsof the spontaneous polarizations Ps1, Ps2 may be both counter-clock wisewhen the array substrate 210 and the color filter substrate 270 areattached facing each other.

Thus, contrary to the first exemplary embodiment, the liquid crystaldirectors 235 and 282 in the first ferroelectric liquid crystal layer233 of the array substrate 210 and the second ferroelectric liquidcrystal layer 280 of the color filter substrate 270 rotatesimultaneously in the same direction, e.g., one of clock wise andcounter-clock wise, by the spontaneous polarizations Ps1, Ps2.Accordingly, the nematic liquid crystal molecules in the twisted nematicliquid crystal layer 290 used as the alignment layer of the twoferroelectric liquid crystal layers 233 and 280 rotate fast in thedirection of rotation of the liquid crystal directors 235 and 282 in thefirst and second ferroelectric liquid crystal layers 233 and 280. Thearray substrate 210 includes a thin film transistor Tr, a pixelelectrode 220, and a first alignment layer 222, and the color filtersubstrate 270 includes a black matrix 274, a color filter layer 271,which includes a plurality of red sub-color filter patterns 271 a, greensub-color filter pattern 271 b and blue sub-color filter patterns 271 c,a common electrode 272 and a second alignment layer 275

As a result, the in-plane switching mode liquid crystal display deviceaccording to the second exemplary embodiment has a faster response speedthan the related art, and further has a faster response speed and anmore improved transmittance than the first exemplary embodiment fixedone of the first and second ferroelectric liquid crystal layers 133 and180 of FIG. 6. Alternatively, the directions of the spontaneouspolarizations Ps1, Ps2 may face to the second alignment layer of thecolor filter substrate, and thus it is obtained the same result that thedirections of the spontaneous polarizations face to the first alignmentlayer.

FIG. 10 is a graph illustrating a relation of a transmittance and asupplied voltage to an in-plane switching mode liquid crystal displaydevice according to the related art and the first and second exemplaryembodiments of the present invention. As shown in FIG. 10, the in-planeswitching mode liquid crystal display device according to the relatedart has a peak of transmittance of about 70% in 7V to 8V, while thein-plane switching mode liquid crystal display device according to thefirst exemplary embodiment of the present invention has a peak oftransmittance of about 40% in 12V nearby. Thus, the transmittanceaccording to the first exemplary embodiment versus related artdecreases. However, the in-plane switching mode liquid crystal displaydevice according to the second exemplary embodiment of the presentinvention has a peak of transmittance of about 80% in 7V to 8V. Thus,the transmittance according to the second exemplary embodiment versusrelated art increases.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the in-plane switching modeliquid crystal display device and the fabrication method thereof of thepresent invention without departing from the spirit or scope of theinvention. Thus, it is intended that the present invention covers themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1. A fabrication method of an in-plane switching mode liquid crystaldisplay device, comprising: forming a pixel electrode on a firstsubstrate; forming a common electrode on a second substrate; forming afirst alignment layer on the pixel electrode; forming a second alignmentlayer on the common electrode; forming a first ferroelectric liquidcrystal layer on the first alignment layer; forming a secondferroelectric liquid crystal layer on the second alignment layer;exposing the first ferroelectric liquid crystal layer to a firstatmosphere to generate a first spontaneous polarization; exposing thesecond ferroelectric liquid crystal layer to a second atmospheredifferent from the first atmosphere to generate a second spontaneouspolarization, a rotational direction of the first ferroelectric liquidcrystal layer with respect to the first alignment layer being differentfrom a rotational direction of the second ferroelectric liquid crystallayer with respect to the second alignment layer; attaching the firstand second substrates with the first and second ferroelectric liquidcrystal layers facing each other; and forming a twisted nematic liquidcrystal layer between the first and second ferroelectric liquid crystallayers.
 2. The method according to claim 1, wherein the first atmosphereincludes air having a lower polarity than the first alignment layer, andthe second atmosphere includes O₂ having a higher polarity than thesecond alignment layer.
 3. The method according to claim 1, wherein thefirst atmosphere includes O₂ having a higher polarity than the firstalignment layer, and the second atmosphere includes air having a lowerpolarity than the second alignment layer.
 4. The method according toclaim 1, wherein the first atmosphere includes a + electric field, andthe second atmosphere includes a − electric field.
 5. The methodaccording to claim 1, wherein the first atmosphere includes a − electricfield and the second atmosphere includes an a + electric field.
 6. Themethod according to claim 1, wherein after attaching the first andsecond substrates, the rotational direction of the first spontaneouspolarization is the same as the rotational direction of the secondspontaneous polarization.
 7. The method according to claim 1, furthercomprising: setting a molecule alignment direction of the firstferroelectric liquid crystal layer to be the same as a moleculealignment direction of the second ferroelectric liquid crystal layer. 8.The method according to claim 1, wherein the forming the firstferroelectric liquid crystal layer and the second ferroelectric liquidcrystal layer includes coating of a ferroelectric liquid crystalmaterial of an isotropic phase using one of a bar coating device, a spincoating device and a slit coating device.
 9. The method according toclaim 8, wherein the coating further includes adding a volatile solventin the ferroelectric liquid crystal of the isotropic phase to decrease aviscosity.